1. Field of the Invention
The present invention is related to an electrical interconnecting structure. More particularly, the present invention is related to an electrical interconnecting structure applied to a circuit board and a fabricating process thereof.
2. Description of Related Art
A conventional circuit board is mainly composed of a plurality of patterned conductive layers and a plurality of dielectric layers alternately stacked to one another. In addition, the patterned conductive layers are electrically connected through a plurality of conductive vias. On the other hand, the circuit board can be further categorized by processes for fabricating the same. Generally, the circuit board is fabricated by performing a laminating process or a build-up process. In most cases, the circuit board with a comparatively low layout density is fabricated by performing the laminating process, while the circuit board with a comparatively high layout density is manufactured by performing the build-up process.
Please refer to FIGS. 1A through 1G which are cross-sectional flowcharts of a process for fabricating a conventional circuit board. As shown in FIG. 1A, non-patterned conductive layers 110a and 110b are respectively disposed on two opposite surfaces of a dielectric layer 100. Here, the dielectric layer 100 can be made of epoxy resin or glass-fiber-containing epoxy resin, while the conductive layers 110a and 110b are made of copper.
In FIG. 1B, a plurality of through holes 112 (only one is depicted) are then formed in the dielectric layer 100 and the non-patterned conductive layers 110a and 110b. A method of forming the through holes 112 includes mechanical drilling or laser ablating.
Next, as shown in FIG. 1C, a conductive wall acting as a conductive through via 114 is formed on respective surfaces of the through holes 112 by electroplating. During the formation of the conductive through via 114 by electroplating, an electroplating layer is formed on the surface of the conductive layer 110a, while another electroplating layer is formed on the surface of the conductive layer 110b. Here, the two electroplating layers serve as a part of the conductive layers 110a and 110b, respectively.
After that, as indicated in FIG. 1D, the non-patterned conductive layers 110a and 110b are patterned by performing photolithography and etching processes, so as to form the patterned conductive layers 110a and 110b. 
With reference to FIG. 1E, the laminating process or the build-up process is then performed for forming dielectric layers 120a and 120b respectively on the patterned conductive layers 110a and 110b. Openings 116a and 116b are then formed on the dielectric layers 120a and 120b by way of mechanical drilling or laser ablating.
As shown in FIG. 1F, the openings 116a and 116b are then filled with a conductive material by electroplating, such that conductive micro vias 118a and 118b are formed. Meanwhile, conductive layers 130a and 130b are formed on the dielectric layers 120a and 120b. Here, the conductive micro vias 118a and 118b and the non-patterned conductive layers 130a and 130b are formed by electroplating.
Afterwards, as indicated in FIG. 1G, the conductive layers 130a and 130b are patterned through implementing the photolithography and etching processes. Next, two solder masks 140a and 140b are respectively formed on the patterned conductive layers 130a and 130b, while the patterned conductive layers 130a and 130b are partially exposed by the two solder masks 140a and 140b. As such, a circuit board 150 is completely formed.
It is known from the aforesaid process for manufacturing the conventional circuit board that the circuit board must be formed by alternately stacking a plurality of patterned conductive layers and a plurality of dielectric layers. However, owing to the limitation on the line width and the pitch of ultra fine wires, the layout density of the circuit board is not able to be further enhanced. Therefore, when the circuits are prone to be equipped with high density and great complexity, the layout density of the conductive layers that are formed by performing the photolithography and etching processes is not likely to be improved. As long as the circuit board is meant to provide more signal transmission paths, the disposition of additional patterned conductive layers in the circuit board is required. As a result, the thickness of the circuit board is inevitably increased.